(a) Field of the Invention
This invention relates to acoustic simulators for simulating signals provided by hydrophones in multi-transducer sonar sounding systems for laboratory or dockside testing of multi-transducer sonar sounding systems. In particular, the invention relates to a system which electrically replaces the hydrophone of a multi-transducer sonar array sounding system.
(b) Discussion of the Prior Art
At present it is very difficult to provide the acoustic signals generated by a large array of hydrophones which accurately represent the ocean bottom and, in real time, apply the signals to an array sounding system for testing the array sounding system. For example, the Sonar Array Sounding System (SASS) currently in operation on board T-AGS 22, 34, 38, and 39 Oceanogragraphic Survey Vessels includes one hundred forty four hydrophones in its receiving array. Thus in order to simulate hydrophone output and test the SASS, one hundred forty four signals must be applied to the SASS during each simulator update. These updates occur on the order of every 2.5 msec and take place for a duration of three to five seconds. Providing this much data to the SASS is a complex operation requiring a very large bandwidth.
The Dummy Hydrophone Array (DHA) was a test set which simulated hydrophone output for testing of SASS. Hydrophone output was simulated in the DHA by providing an echo signal having a constant amplitude and varying phase. This technique was used to select a single one of the ninety one possible beams which a SASS produces from the hydrophone outputs. The selected beam was in either an on state or an off state. Since the DHA could thus only simulate one beam at a time it could troubleshoot the beam forming function but could not produce a composite acoustic output which accurately represented a complete slice of the ocean bottom.
Another test system available to troubleshoot a SASS was the Sonar Test Signal Generator (STSG). The STSG was used to simulate minimum and maximum pulse-widths of bottom return signals. The STSG produced only one output signal which was injected across all one hundred forty four hydrophones in parallel. However, in order to accurately reproduce the ocean bottom, one hundred forty four distinct, phase related, acoustic signals must be applied to the SASS rather than a single signal injected in parallel across all of on hundred forty four hydrophones. Therefore the STSG could not be used to provide a true acoustic simulation.
U.S. Pat. No. 3,363,045, issued to Pommerening on Jan. 9, 1968, teaches a sonar target simulator in which forty eight channels of data were simulated from the output of a sound reproduction system. In the simulator of Pommerening a single signal was applied to a plurality of delay lines and pulse shapers for simulating the signals of the forty eight channels. Since these forty eight channels all contained data from the same data point, the result could not represent real bottom data.
U.S. Pat. No. 3,676,565, issued to Rowe on July 11, 1972, teaches a method for synthesizing a time domain waveform. Time domain waveforms were provided with improved resolution in this method by processing them with Fourier transforms to obtain time domain data which was added and passed through a digital to analog converter to provide analog time domain waveforms. However performing these transforms on all signals from each hydrophone for each update of the simulation for a large number of hydrophones is too time consuming. Additionally, the resolution with which a SASS could be tested from these waveforms was still dependent on the resolution of the digital data.
In view of the difficulty in testing a SASS with signals representative of a real ocean bottom, cumbersome methods of testing SASS improvements at sea were used. Incremental changes, on a not to interfere basis, were made to a SASS and tested during sea trials. If the changes did not perform as expected, they were removed so that the ship could perform its responsibilities with fully operational equipment. If a major hardware update was required, both the new hardware and the existing hardware were installed in parallel and both systems were prepared for full operation. Thus the ship could use the existing hardware if the new hardware did not perform as expected. These constraints produced long delays and additional costs.